Odorant composition for liquefied petroleum gases

ABSTRACT

IMPROVED ODORANT COMPOSITION FOR LIQUEFIED PETROLEUM GASES INCLUDES AN AZEOTROPIC MIXTURE OF AN ORGANIC SULFUR COMPOUND SELECTED FROM THE GROUP CONSISTING OF ETHYL MERCAPTAN AND DIMETHYL SULFIDE AND AT LEAST ONE RELATIVELY NON-ODOROUS, CHEMICALLY INERT MATERIAL, SUCH AS METHYL FORMATE, N-PENTANE, IOSPENTANE, AMYLENE, ISOAMYLENE, CHLOROPROPANE AND THE LIKE, CAPABLE OF FORMING A MINIMUM BOILING POINT AZEOTROPE WITH THE ORGANIC SULFUR ODORANT COMPOUND PRESENT.

United States Patent Ofi 3,826,631 Patented July 30, 1974 ice 3,826,631 ODORANT COMPOSITION FOR LIQUEFIED PETROLEUM GASES Ashley Dwight Nevers, King of Prussia, Pa. Pennwalt (1Io1rpo)ration, 1012 Pennwalt Bldg., Philadelphia, Pa.

9 02 No Drawing. Continuation-impart of abandoned application Ser. No. 153,853, June 16, 1971. This application Mar. 27, 1972, Ser. No. 238,541

Int. C1. C101 1 24 US. Cl. 44-52 9 Claims ABSTRACT OF THE DISCLOSURE Improved odorant composition for liquefied petroleum gases includes an azeotropic mixture of an organic sulfur compound selected from the group consisting of ethyl mercaptan and dimethyl sulfide and at least one relatively non-odorous, chemically inert material, such as methyl formate, n-pentane, isopeutane, amylene, isoamylene, chloropropane and the like, capable of forming a minimum boiling point azeotrope with the organic sulfur odorant compound present.

This application is a continuation-in-part of my prior application, Ser. No. 153,853, filed June 16, 1971, now abandoned.

This invention relates to odorants that are incorporated with combustible fuel gases in order to serve as a warning agent by means of Which leaks or open gas cocks can be detected and immediately brought to the attention of one in the vicinity. More particularly, this invention relates to a novel and improved composition containing ethyl mercaptan and/ or dimethyl sulfide as an odorant for liquefied petroleum gases, such as propane or butane.

Commercial liquefied petroleum gas (LPG) is customarily odorized with either ethyl mercaptan or thiophane at a rate of about one pound of odorant per 10,000

gallons of LPG. However, both ethyl mercaptan and thiophane have the serious disadvantage of being higherboiling than propane or butane so that the odorant concentration is not uniform in the vapors evolving from the odorized liquid. That is, the gas is always much leaner than the liquid with respect to odorant concentration, and the odorant concentration may vary over a 20 to 50 fold range as evaporation continues.

Although ethyl mercaptan is the leading commercial odorant in terms of unit weight consumed, it is subject to an inherent objection as the LPG containers or vessels are successively exhausted, refilled and reused. Not only does the accumulation of ethyl mercaptan in the spent cylinder residues produce a progressively increasing atrocious stench in the expended cylinder, but also a ferrous mercaptide/sulfide complex builds up as a result of a reaction with the container walls. This ferrous mercaptide/ sulfide residue is frequently susceptible to spontaneous ignition upon exposure to air.

While attempts have been made to locate and test candidate odorants which form an azeotrope with propane and butane in order to yield a uniform odorant concentration in the vapor, to date no azeotrope has been found having the desired composition and odor intensity. Furthermore, reference to Horsely, Azeotropic Data, Am.

Chem. Soc., Advances in Chemistry, Series No. 6, 1952 mia in which certain people are completely insensitive to odors that are readily perceptible to most persons in the general population. In this regard, it is considered that current LPG odorants, especially ethyl mercaptan, are less than ideal in effect upon some people. That is, even though the smell is readily perceived, such odor is not always identifiable by the observer as one characterisic of gas.

It is therefore an object of this invention to provide an improved odorant composition in which the foregoing problems are substantially eliminated or reduced.

Other objects of this invention are to provide an improved odorant of the character described which is easily and economically produced, effective in application and highly efiicient in operation.

In essence, the odorant composition of the instant invention comprises in combination with LPG an azeotropic mixture of (l) ethyl mercaptan and/or dimethyl sulfide and (2) a chemically inert, relatively non-odorous organic material which is capable of forming a minimum boiling point azeotrope with which ever organic sulfur compound of (l) is present. The group (2) compounds are hereinafter referred to as azeotrope-formers and should be present in an amount in excess of that which is required to form the azeotrope itself.

The azeotropes may either be binary or ternary. The binary compositions are a mixture of either ethyl mercaptan or dimethyl sulfide with the azeotrope-former, the latter constituent acting as an azeotroping agent with the sulfur-containing organic odorant. The ternary compositions comprise both ethyl mercaptan and dimethyl sulfide in combination with the azeotrope-former.

The azeotrope-former itself may either be a single compound or a mixture of compounds, typically having a boiling point ranging between 22 C. to about 37 C., and capable of forming an azeotrope with either ethyl mercaptan, dimethyl sulfide, or both. By virtue of economy and their compatibility with both the active organic sulfur odorant compounds and the propane-butane system itself, hydrocarbons in the C range which satisfy the boiling point range requirements are suitable azeotrope formers for the purpose of this invention. The hydrocarbons may be straight chain, branched chain, saturated or unsaturated.

The effective azeotrope-formers are not limited to hydrocarbons, but may include halogenated hydrocarbons, alcohols, ethers, esters, or other compatible organic compounds which are capable of forming minimium boiling azeotropes with ethyl mercaptan,, dimethyl sulfide or both. However, the azeotroping agent should be substantially chemically inert with respect to the other ingredients in the system and relatively non-odorus. In particular, methyl formate is a preferred azeotrope-former for both ethyl mercaptan and dimethyl sulfide. Another preferred azeotroping agent for either or both of the sulfur odorant compounds named is isopentane. Yet still another preferred azeotrope-former is amylene, for example, a mixure of amylenes available as a side stream from certain petroleum refining operations. In general, it is unnecessary to have a high degree of purity for the azeotrope-former utilized. The commercial grades are of satisfactory purity and are usually preferred for reasons of economy.

The preferred odorant compositions are not the exact composition of the azeotrope itself but contain some azeotrope-former in excess of the amount required to form the azeotrope alone. The preferred azeotrope-formers are relatively non-odorous and chemically inert and by their presence in slight excess, such compounds become concentrated in the evaporation residues from an LPG vessel. Therefore, the residual liquid in this instance is not odorant-rich but becomes progressively richer in low-odor azeotrope-former as evaporation proceeds. This minimizes the aforementioned problems of pyrophoric mer- Jcaptide/ sulfide formation andprogressive accumulation of obnoxious residual odorant A further advantage of the compositions of this invention is that the azeotropes, having lower boiling points than the individual, odorant compounds themselves, will have an improved vapor-liquid distribution during the evaporation. of the liquid from a full container of odorized ,LPG. The vapor concentration of odorant still increases from beginning to end of the evaporation but the over-all range of concentrations is less, there being more uniform odorization from the beginning to the end of a cylinder.

The following examples will serve to illustrate the present invention.

EXAMPLE IIsopentane azeotropes Percent by weight Isopentane 73.5 Ethyl mercaptan 22.5 Dimethyl sulfide 4.0

The boiling point of the foregoing ternary azeotrope was 25.1 C.

*(b) Binary azeotropes are known for both the isopentane-ethyl mercaptan system and the isopentane-dimethyl sulfide system as follows:

Percent by weight Isopentane 71-75 (1) Ethyl mercaptan 29 (2) Dimethyl sulfide 25 (azeotrope B.P. C.-25.7-26.6)

It is to be observed that the approximate C. reduction in boiling point from that of the odorant (ethyl mercaptan B.P.=35.0 C. and dimethyl sulfide=37.8 C.) produces asubstantial advantage in terms of more uniform odorant concentration in the vapor evloved from the odorized propane-butane liquid. It is also apparent that the lower boiling point of the ternary system in Example I(a) renders the latter somewhat preferable over either of the binary examples I(b)(l) or I(b)(2). However, a slight excess of isopentane in any of the above examples is preferred for this invention in that the residual liquid would be richer in the low-odor azeotrope former. In the ternary composition of Example I(a) some excess dimethylsulfide is also advantageous since it would maintain gassy odor to the end of the vessel without an overpowering increased intensity or accumulation of pyrophoric residues. The intent is to have an excess of either dimethylsulfide or azeotropic agent but not excess ethyl mercaptan.

EXAMPLE IIMethyl formate azeotropes (a) A ternary azeotrope using methyl formate (B.P. 31.7 C.) was isolated by fractional distillation of an ethyl mercaptan-dimethyl sulfide-methyl formate system Analysis for mercaptan sulfur and total sulfur produced the following calculated composition for the ternary -The boiling point of the foregoing ternary azeotrope was 28.8 C.

(b) The literature (Horsley supra) has listed two binary azeotropes involving methyl formate, as follows:

Percent by weight Methyl formate 30.0-62.0 (1) Ethyl mercaptan 70.0 (2) Dimethyl sulfide 38.0 (azeotrope B.P. C.27.029.0)

EXAMPLE IIIn-pentane azeotropes n-pentane 49.0-55.0 Ethyl mercaptan 51.0 Dimethyl sulfide 45.0

(azeotrope B.P. C.30.4633.5)

EXAMPLE IV'Is0amylenes azeotropes Still another ternary azeotrope has been observed using isoamylene as the azeotrope-former. The isoamylene used was a mixture used commercially in the alkylation of phenol to form tertiary amylphenols and it contained about 96% of a mixture of 3-methyl butene-2 and 2-methyl butene-l. (3-methyl butene-l does not form an azeotrope in mixtures with ethyl mercapan and dimethyl sulfide.) The isoamylene ternary has a boiling point of 34 C. and its composition has not been determined.

Any of the above binary and ternary azeotropes may be used as LPG odorants having improved vapor-liquid equilibrium characteristics. Improvement with respect to residual odor and chemical inertness of the residual liquid at the end of a cylinder is imparted by admixing additional small quantities of azeotrope-former and, optionally, dimethyl sulfide. The latter compound possessesthe advantage of a different odor character (less obnoxious than ethyl mercaptan) and little or no reactivity with the ferrous metal surfaces ofthe container.

EXAMPLE VTernary azeotrope with excess isopentane and dimethyl sulfide To compare the relative evaporation performance of one of the novel compositions of this invention against a standard system odorized with ethyl mercaptan, two separate systems of odorized propane were prepared. In one portion of the experiment, instrument grade propane was odorized on a laboratory scale with commercial grade ethyl mercaptan in a steel container fitted with a vapor discharge valve. In another portion of the experiment, instrument grade propane was odorized in a similar steel container with an azeotropic odorant utilizing one of the examples employing an azeotrope agent. The azeotropic odorant used in the test had a composition of 74.0% isopentane, 21.1% ethyl mercaptan and 4.9% dimethyl sulfide.

Each of the vessels of experimentally odorized liquid was successively connected through thevalve to a pressure regulator feeding the vapor to a Barton #286 titrator. The model 286 Barton titrator is an electronic instrument which function's by the electrolytic titration of sulfur compounds with high sensitivity. The, reaction in an electrolytic cell causes generation of free bromine from HBr, and the free bromine is stoichiometrically reacted with the sulfur compounds passing through the system. Electrolysis current is so controlled that it is directly proportional to the sulfur compounds present whereby electrolysis current is read out on a recording chart. A suitable conversion factor can then be applied to the chart reading and the quantity of sulfur compounds calculated directly. In the present experiment, the objective was to determine the concentration ratio of sulfur compound in.

the'va'por at the-initial evaporation of acylinder to the concentration thereof at the end of a cylinder. Therefore, it was of no interest to compute the actual concentrations of the compound itself and noconversion factor was applied. However; the ratio of scale reading was taken to ihdicate the concentration change that occurred for each odorant during the evaporation. The nature of the laboratory manipnlatio'n did not 'permit exact control of the initial odorant concentration in each of the experimental liquid'sso the starting concentrations were different ineach case. "Nevertheless; this does not affect the validity of the determination of the concentration ratio from the beginning'to the end of the evaporation.

In separate experiments each system was allowed to evaporate at .room temperature "over a period of several hours while the odor ant concentration in the vapor was measured by the Barton titrator. The results obtained are as follows 1 remained from the original charge. Each cylinder was then recharged with a mixture approximately its initial composition, odor observations were repeated and the contents again allowed to evaporate until less than a 10% residue remained. Odor observations were conducted on this residue. The processwas then repeated for another loading and evaporation cycle. The total number of observers in any given test varied from 17 to 22. Odor observations are tabulated in Table II.

TABLE II Blend EtSH No selec- Total Filling cycle cylinder Cylinder 1 Cylinder 2 tion observers I. Initial vapor:

1 Total times selected. 12 4 1 Stronger.- 7 0 1 Weaker- 1 3 1. Unclass 4 1 2 Total times selected. 13 3 2 Stronger 8 1 2 Weaker. 2 1 2 Unclassified. 3 1 3 Total times selected- 7 5 3 ranger 4 1 .3 Weaken. 2 4 3 Unclassified-.. 1 0 II. Residue 1 0% evap.

2 Total times selected. 15 4 2 Stronger 14 l 2 eaker. 0 2 2 Unclassified 1 1 .3 Total times selected. 15 2 3 Stronger 14 0 3 Weaker. l 2 3 Unclassified 0 0 TABLE I As is typical in this type of test, the observers were not Relative evaporation behavior t oderants from propane (EtSH vs.

"isopcntane'azeotrope) Net scale div. (Barton) Isopentane EtSH azeotrope Wt. ercent ro ane eva orated:

295 p p p 23 5-6 5. 8 95.5 510 116 Concentration factor:

95/5% evap 17.0 (510/30) 14.5 (116/8) 95/2.5% evap 22.2 (510/23) 19.3 (116/6) Percent Isopentane 74.0 Ethyl mercaptan 21.1 Dimethyl sulfide 4.9

Two individual stainless steel cylinders were loaded with propane and odorized with the foregoing blend such that the concentration of the blend in propane was 95 ppm. in each cylinder. In each cylinder, the active odorant (ethyl mercaptan and dimethyl sulfide combined) was accordingly 24.7 ppm. Still a third cylinder was charged unanimous but a significant majority (14-15 out of 20) selected the ethyl mercaptan as being stronger in the residue after or more evaporation. This effect did not occur in the initial vapor. Thus a composition in accord with the concept of this invention results in a reduction of obnoxious odor in evaporation residues by comparison with the use of ethyl mercaptan alone.

Two more filling and evaporation cycles were carried out with each of the three cylinders.

The following table summarizes the concentration of added odorant and the percentage of evaporation residue left after each of the five loading and evaporation cycles:

1 g; added to "EtSH Cylinder"; Blend added to "Blend Cylinders The residues after the fifth cycle were analyzed with the following results:

Analysis EtSH, DMS, p.p.m. p.p.m.

EtSH cylinder 113 0 Blend cylinder 1-. 26 29 Blend cylinder 2 60 75 It is clear from this analysis that (1) when ethyl mercaptan is used as the sole odorant, it accumulates to a much larger degree in the evaporation residues than when it is used as a component of a blend meeting the specifications of this invention, and (2) when the odorant blend is used, the residues are selectively enriched with respect to DMS and leaner with respect to ethyl mercaptan. The ratio DMS/EtSH has increased from 0.23 in the original odorant blend to an average of 1.2 in the residue after five loading and evaporation cycles.

The dilference in the results for blend cylinder 1 and blend cylinder 2 are accounted for by the differing percentage residues in the five consecutive evaporation cycles.

Other tests were conducted to demonstrate the effect of the above blend on the composition of the vapor in an early stage of evaporation from odorized propane. Propane odorized with this blend was analyzed for initial liquid composition, and was then allowed to evaporate slowly. The vapor was analyzed when 1% of the liquid had evaporated. Control experiments were performed in a similar way on propane odorized with ethyl mercaptan alone, and on propane odorized with dimethyl sulfide alone. To provide a uniform basis for comparison, the vapor compositions after 1% evaporation was expressed as percentages of the concentration of odorant in the initial liquid.

Vapor concentration at 1% Thus the use of an odorant 'blend representing the practice of the present invention resulted in a relative enrichment, in an early stage of evaporation, of 77% in the case of dimethyl sulfide; and 58.5% in the case of ethyl mercaptan, i.e.

Percent enrichment DMS via blend: (19.1 10.8) 10.8

Percent enrichment EtSH via blend: 16.5- 10.4) 10.4

Although this invention has been described in considerable detail, such description is intended as being illustrative rather than limiting, since the invention may be vari ously embodied, and the scope of the inventionris to be determined as claimed.

What is claimed is:

1. In combination with liquefiied petroleumgas, an odorant composition consisting essentially of (A) at least one organic sulfur compound selected from the group consisting of ethyl mercaptan and dimethyl sulfide, and (B) a relatively non-odorous azeotropic agent, .said azeotropic agent being an organic compound which is miscible with and forms a minimum boiling point azeos trope with (A). I, p I I 2.'The odorant composition of Claim 1 wherein the azeotropic agent is present in excess of the amount required to form an azeotrope with (A).

3. The azeotrope of Claim 1 wherein. the system isv ternary. v

4. The odorant composition of Claim 3 wherein-dimethyl sulfide is present in excess over the amount reqggred to form an azeotrope with ethyl mercaptan and 5. The azeotrope of Claim 1 wherein the azeotropi agent is selected from the group consisting of methyl formate, n-pentane, isopentane, and amylene.

6. The az'eotrope. of Claim 5 wherein the amylene comprises a mixture of S-methyl butene-Z and Z-methyl butene-l.

7. azeotropic ternary composition comprising ethyl mercaptan, dimethyl sulfide and an organic azeotropeformiiig compound selected from the class consisting of methyhformate, n-pentane, isopentane and amylene.

8...,The composition of Claim 7 wherein the dimethyl sulfide is present in excess of azeotropic proportions.

9. The composition of Claim 7 wherein the organic azeotrope-forming compound is present in excess of azeotropic proportions.

References Cited UNITED STATES PATENTS 2,032,431 3/1936 Odell 44-59 1,9 4,175 1/1934 Frey 44-59 1,565,935 12/1935 Harris 48-496 F M DANIEL E. WYMAN, Primary Examiner Y. H. SMITH, Assistant Examiner 

